Etching methods have been developed for making high frequency fundamental mode resonators at frequencies above 1 GHz. One of the major problems with such high frequency resonators is the adjustment of the resonator frequency to a desired frequency. For example, to change the frequency of an aluminum plated 200 MHz fundamental mode resonator by .+-.1000 Hz, the aluminum thickness on each side must be changed by .+-.2 .degree.A (or by .+-.4 .degree.A on one side only). This change is very difficult to produce by conventional techniques.
In U.S. Pat. No. 4,130,771, issued Dec. 19, 1978, V. E. Bottom proposed the use of anodic oxidation as a method of adjusting the frequency of quartz crystal resonators. The main drawback of this method is that, since it is a wet chemical method, it is not possible to monitor the resonator's frequency during the anodization. Moreover, the anodization solution contaminates the electrodes and thus can lead to undersirable aging.
In the paper "Frequency Tuning of Quartz Resonators by Plasma Anodization", by J. J. H. Reche, in the Proc. 33rd Ann. Symposium on Frequency Control, on pages 299-303, 1978, the use of plasma anodization is proposed for adjusting the frequency of quartz crystal resonators. Although this method overcomes the monitoring and contamination problems inherent in the wet anodization method, the method requires expensive equipment, and it subjects the resonators to undesirable levels of ionizing radiation.
In U.S. Pat. No. 4,472,652, issued Sept. 18, 1984, J. C. Brice and J. E. Curran proposed the use of thermal oxidation of aluminum electrodes, at a temperature between 250.degree. C. and 500.degree. C., for adjusting the frequency of quartz crystal resonators and for passivating the electrodes. The main drawback of this method is that the required high temperatures can be detrimental. For example, the epoxy in most of the silver filled epoxies used to bond the resonator plates to the mounting clips start to break down chemically at about 200.degree. C. The high temperature treatment thus causes excessive outgassing and a significant weakening of the bond when the method is performed after the resonators are bonded. The method can be used before the resonators are bonded, however, in that case the electroded blanks must undergo undersirable handling during the bonding process.